Yealink SIP-T27P User Manual page 179

The default value is 1. In this example, the value is set to 2.
Click Confirm to accept the change.
4.
The phone A will automatically configure the line keys from the first unused DSS key, according
to the configured number in the Number of line key field.
To configure the shared line settings and the number of line key on phone B via web user
interface:
Register the alternate account 4603_1.
1.
Advanced Phone Features
167